Complex electronic devices such as two way radios, cellular telephones, and computers are increasingly becoming portable as electronics are integrated into smaller, more efficient embodiments. Concurrently, there has been an increase in demand for quality battery systems to power the portable devices. For devices that have a high power demand, such as cellular phones and portable computers, rechargeable battery systems, particularly nickel-cadmium and nickel metal hydride systems, are the most economical choice. However, the use of batteries has presented designers of such devices with an interesting challenge; without the benefit of a regulated power supply, the device must operate over an input voltage range, rather than at one particular voltage. Additionally, the battery voltage is continuously changing as different subsystems of the device turn on and off. The problem, then, is to get the device to operate consistently while the bias voltage of the electronics is continuously changing.
Certainly the means and knowledge exist to regulate battery voltage, either in the device or in the battery itself. Integrated linear voltage regulators that transform the raw battery voltage into a stable voltage for the device are available. Linear regulators, however, in light of the fact that operation time is a critical market feature, are prohibitively inefficient for all but very low current sub-systems of the device. Switched mode regulators can provide a more efficient means of regulating battery voltage, but cost and complexity also prohibit the use of such means to low current sub-systems. Therefore, the designer of portable electronic devices is left to contend with a battery voltage that may change as much as 100% from a fully discharged state to a peak voltage while being charged.
Since recharging may take an hour or more, it is advantageous to allow the device to operate while the battery is being recharged. Consequently, the device is exposed to the charger voltage. As mentioned previously, the peak voltage necessary for optimum charging can be quite high. The solutions arrived at to allow the device to operate efficiently over the operating voltage range of the battery are such that the device may be irreversibly damaged if peak charger voltage is applied. This is particularly true in hand held communications devices where the radio frequency power amplifier is typically connected directly across the raw battery voltage. Knowledgeable battery system designers would avoid this scenario by limiting the charger voltage to protect the device, sacrificing charging time as a compromise. However, third party manufacturers of battery chargers would likely not be aware of such limitations, and would make and sell chargers that could damage the device. Should this occur, the user would perceive that the device is defective, unaware that the charger caused the problem.
Another instance where the device would be exposed to excessive charger voltages is emerging as a result of the use of lithium-ion based batteries. Batteries based on such cells provide a significant advantage over nickel based systems in energy density, and afford the user more operation time, less weight, or a combination of both. However, these cells are sensitive to voltage, and, for safety reasons, must not be charged above a certain safety threshold voltage. To insure safety is maintained, it is necessary to provide a safety switch in series with the cells to interrupt a charge current, should it become necessary. The safety switch is controlled by a circuit responsive to cell voltage and activates the safety switch when the cell voltage reaches the safety threshold voltage. In a charger designed for lithium ion batteries, this would be a rare occurrence. However, given the fact that many customers would be unwilling to buy a new charger just to use lithium ion batteries, it would be advantageous if a lithium ion battery were designed in such a way that it could be charged in existing nickel-based system chargers. The safety switch of a lithium ion battery charged in a nickel-based system charger would be switched off every recharge cycle because the safety threshold voltage of a lithium ion battery is roughly two thirds of the voltage nickel batteries for the same applications can achieve during charging. When the safety switch switches off, a nickel-based system charger will raise its output voltage in an attempt to maintain constant current regulation. Since the cells and safety switch form a series structure in parallel with the device, the maximum charger voltage would be applied to the device that the battery is powering.
Therefore there exists a need in both nickel-based battery systems, and more particularly lithium ion batteries, to protect the associated device from excessive voltages produced by a charger when the battery is being recharged. Such means should reside in the battery pack so that the battery may be used in any charger.